In chemistry class, the girl to boy ratio is $9$ to $6$. If there are $45$ girls, how many boys are there?
A ratio of $9$ girls to $6$ boys means that there is a group of $9$ girls for every group of $6$ boys. If there are $45$ girls, then there are $5$ groups of $9$ girls each. Because we know the ratio is $9:6$ , there must be $5$ groups of $6$ boys each. There is a total of $30$ boys in chemistry class.